The various properties you can access in a native GroupWise task object are documented by Novell in the Object API. I don’t think they actually expose the completed date – just the completion status.
The new GroupWise 6.5 categories can also be accessed via the Object API. Note on the link above the Task object (as well as Mail, Notes, Appointments etc) all have a Categories() property. The Categories property provides a Categories collection. You use this collection to access the categories a message has been assigned to.
For example, given a task object, you coulds access the first category name by doing something like (untested code follows):
I hope this helps.